What is the topic sentence of the paragraph below? Monarch butterflies make an incredible journey, though no single butterfly lives for the whole trip. In the fall, the monarchs in the United States migrate south to Mexico or California. In the spring, butterflies return to the northern states—but these are the children or grandchildren of the butterflies who left the previous winter. Scientists still don’t know how the monarchs know how to return to their ancestral home.

A. In the spring, butterflies return to the northern states—but these are the children or grandchildren of the butterflies who left the previous winter.

B. Scientists still don’t know how the monarchs know how to return to their ancestral home.

C. Monarch butterflies make an incredible journey, though no single butterfly lives for the whole trip.

D. In the fall, the monarchs in the United States migrate south to Mexico or California.

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